If you were outside in Key West between 4:00-6:00 p.m. on Wednesday, May 5, 2004, chances are that you noticed at least one of numerous funnel clouds and waterspouts that developed southwest, west, and north of the island. Several waterspouts were observed over the waters between Key West and the Marquesas Keys by boaters, pilots, residents, and tourists in and around Key West. National Weather Service observers spotted at least 11 different funnels late in the afternoon on the 5th from their temporary facility at Key West International Airport (groundbreaking on a new, permanent facility will commence June 1, 2004 at a government lot located on the east end of White street between United and Seminary Streets). Incidentally, June 1 marks the official beginning of the Atlantic Hurricane Season.
If you have lived in the Florida Keys for a few years, then you probably have seen at least one or two waterspouts during the rainy season, which runs from late May through early November in the Keys. If you work on the water, then you no doubt have seen many waterspouts during the course of your offshore operations. So, what are these "waterspouts" exactly? Well, a waterspout is defined by the American Meteorological Society as simply a "tornado over water", and a tornado is defined as "a violently rotating column of air, in contact with the ground, either pendant from a cumuliform cloud or underneath a cumiliform cloud, and often (but not always) visible as a funnel cloud". Not all tornadoes are the same with respect to size, intensity, duration, translational speed, or even genesis mechanism. A rough estimate of tornado intensity can be obtained through the Fujita Tornado Damage Scale, which was developed by the late University of Chicago meteorology Professor T. Theodore Fujita. Most tornadoes that have hit the Florida Keys since about 1950 have been either F0 or F1 on the Fujita Scale, with a few F2 tornadoes recorded. The strongest (F3, F4 and F5) tornadoes have never been observed in the Keys (although there have been a few F3 and F4 tornadoes recorded over mainland Florida). The rarity of these stronger twisters over the subtropics is due to the fact that in order for these stronger tornadoes to form, they typically need the rapid changes in wind speed and direction with height associated with the jet stream and large-scale midlatitude storm systems. Occasionally, conditions do become favorable over the Florida peninsula for violent tornadoes, as was the case in the Central Florida Tornado Outbreak of February 22-23, 1998.
Most Florida Keys waterspouts (including those which occurred on May 5, 2004) develop from rapidly building cumulus congestus cloud lines. These cloud lines most often form over the Lower Keys during hot and humid summer afternoons. Warm, moist air begins rising over the heated islands and shallow warm waters, and then cooler air from over the deeper and cooler waters north and south of the island chain moves toward the islands to take the place of the rising air. Air in the lower portion of the atmosphere near the surface then converges along or immediately adjacent to the keys, producing a narrow zone of horizontal "wind shear" beneath a line of rapidly building cumulus clouds. The atmosphere behaves as a type of fluid, and air tends to develop "spin" in areas of horizontal wind shear. If one of the stronger cumulus clouds moves over an area of spin at the surface, the updraft associated with the developing cloud will rapidly stretch and intensify the area of spin to tornadic strength, from sea surface to cloud base. When this happens, a funnel cloud will be seen extending from the cloud base, sometimes all the way to the sea surface. Well-developed waterspouts will have a ring of sea spray swirling at their base on the surface of the ocean. If you see this "spray ring", the winds are likely at least gale force (34 knots) over a small area, and you would be wise to stay away as this wind will have sufficient force to overturn most small watercraft.
Waterspout season usually begins in May and ends in October. It is during this time of year that the atmosphere often possesses the ingredients for waterspout formation of the type described in the above paragraph. Waterspouts (and tornadoes) may also form in the rainbands of tropical cyclones, as well as in association with winter and spring squall lines that precede stronger cold fronts.
National Weather Service meteorologists at the Key West office will issue a "Special Marine Warning" if a waterspout has been sighted by National Weather Service observers or storm spotters, or if a doppler radar signature of a waterspout is noted. If a waterspout is expected to move across a populated land mass, then a "Tornado Warning" will be issued. If conditions look increasingly favorable for waterspout formation, a "Marine Weather Statement" or "Short Term Forecast" will be issued to provide details on the developing weather situation.
Have a safe waterspout season!